/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core;

/**
 * Internal helper class used to find the Java/JVM version
 * that Spring is operating on, to allow for automatically
 * adapting to the present platform's capabilities.
 *
 * <p>Note that Spring requires JVM 1.5 or higher, as of Spring 3.0.
 *
 * 得到Spring运行所在的Java版本
 * 从而更好的适应当前运行的平台
 * 在Spring3.0之后，它需要JVM1.5或者更高版本
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Rick Evans
 */
public abstract class JdkVersion {

    /**
     * Constant identifying the 1.3.x JVM (JDK 1.3).
     */
    public static final int JAVA_13 = 0;

    /**
     * Constant identifying the 1.4.x JVM (J2SE 1.4).
     */
    public static final int JAVA_14 = 1;

    /**
     * Constant identifying the 1.5 JVM (Java 5).
     */
    public static final int JAVA_15 = 2;

    /**
     * Constant identifying the 1.6 JVM (Java 6).
     */
    public static final int JAVA_16 = 3;

    /**
     * Constant identifying the 1.7 JVM (Java 7).
     */
    public static final int JAVA_17 = 4;

    /**
     * Constant identifying the 1.8 JVM (Java 8).
     */
    public static final int JAVA_18 = 5;


    /**
     * 当前的Java版本
     */
    private static final String javaVersion;

    /**
     * 主版本号
     */
    private static final int majorJavaVersion;

    /**
     * 首先获取到Java的版本
     */
    static {
        /**
         * 通过获取环境变量的值来实现
         */
        javaVersion = System.getProperty("java.version");
        // version String should look like "1.4.2_10"
        if (javaVersion.contains("1.8.")) {
            majorJavaVersion = JAVA_18;
        } else if (javaVersion.contains("1.7.")) {
            majorJavaVersion = JAVA_17;
        } else if (javaVersion.contains("1.6.")) {
            majorJavaVersion = JAVA_16;
        } else {
            // else leave 1.5 as default (it's either 1.5 or unknown)
            // 默认是Java 1.5，即使它有可能不是
            majorJavaVersion = JAVA_15;
        }
    }


    /**
     * Return the full Java version string, as returned by
     * {@code System.getProperty("java.version")}.
     *
     * 获取当前的Java版本号
     *
     * @return the full Java version string
     * @see System#getProperty(String)
     */
    public static String getJavaVersion() {
        return javaVersion;
    }

    /**
     * Get the major version code. This means we can do things like
     *
     * 获取主版本号
     *
     * {@code if (getMajorJavaVersion() >= JAVA_17)}.
     * @return a code comparable to the JAVA_XX codes in this class
     * @see #JAVA_13
     * @see #JAVA_14
     * @see #JAVA_15
     * @see #JAVA_16
     * @see #JAVA_17
     */
    public static int getMajorJavaVersion() {
        return majorJavaVersion;
    }


    /**
     * Convenience method to determine if the current JVM is at least Java 1.4.
     *
     * 判断当前版本是否高于1.4
     *
     * @return {@code true} if the current JVM is at least Java 1.4
     * @deprecated as of Spring 3.0 which requires Java 1.5+
     * @see #getMajorJavaVersion()
     * @see #JAVA_14
     * @see #JAVA_15
     * @see #JAVA_16
     * @see #JAVA_17
     */
    @Deprecated
    public static boolean isAtLeastJava14() {
        return true;
    }

    /**
     * Convenience method to determine if the current JVM is at least
     * Java 1.5 (Java 5).
     *
     * 判断当前版本是否高于1.5
     *
     * @return {@code true} if the current JVM is at least Java 1.5
     * @deprecated as of Spring 3.0 which requires Java 1.5+
     * @see #getMajorJavaVersion()
     * @see #JAVA_15
     * @see #JAVA_16
     * @see #JAVA_17
     */
    @Deprecated
    public static boolean isAtLeastJava15() {
        return true;
    }

    /**
     * Convenience method to determine if the current JVM is at least
     * Java 1.6 (Java 6).
     *
     * 判断当前版本是否高于1.6
     *
     *
     * @return {@code true} if the current JVM is at least Java 1.6
     * @deprecated as of Spring 3.0, in favor of reflective checks for
     * the specific Java 1.6 classes of interest
     * @see #getMajorJavaVersion()
     * @see #JAVA_16
     * @see #JAVA_17
     */
    @Deprecated
    public static boolean isAtLeastJava16() {
        return (majorJavaVersion >= JAVA_16);
    }

}
